This invention relates to a conveyor screw, especially but not exclusively for use in a decanter centrifuge, comprising a substantially continuous row of generally quadrilateral wear-resistant elements mounted on the operative surface of the conveyor screw along the outer contour thereof with their outer longitudinal edge faces following broadly the contour of the screw surface and providing beyond that contour and with their end faces extending generally in radial directions and in abutting relationship, said wear-resistant elements being retained on the conveyor screw by means of element retaining members secured to the conveyor screw between consecutive elements.
The wear-resistant elements, which prevent erosion of the screw when the goods being transported are highly abrasive, are generally made of carbide, such as tungsten carbide, or ceramic material, e.g. aluminium oxide, i.e. materials whose thermal coefficient of expansion differs considerably from the coefficient of expansion of the screw material, normally steel. Consequently, a direct securing of the elements to the screw by brazing or glueing has proven to be unsuited, and instead one has looked for suitable methods of mechanically securing the wear-resistant elements to the surface of the screw.
The published German patent application No. 25 56 671 (which claims priority from a U.S. patent application Ser. No. 533,198 filed Dec. 16, 1974 now U.S. Pat. No. 3,977,515) discloses a conveyor screw of the kind referred to above in which complementary dovetail formations are provided on the underside of each wear-resistant element, i.e. the side facing the screw surface, and on element retaining blocks welded to the screw surface, respectively. In plan view the dovetail formations on the wear-resistant elements converge outwardly from the screw axis, while on the retaining blocks they diverge outwardly. The wear-resistant elements are sequentially inserted in the wedge-shaped spaces defined between consecutive retaining blocks from the inner end of each space, and they are then driven outwardly until they are held securely by the engagement between the opposed dovetail formations. The wedge action creates inherent tensile and bending stresses in the zones of the wear-resistant elements adjacent the dovetail formations. If, during operation, the conveyor screw rotates at high speed, such as in a centrifuge, the wear-resistant elements are subjected to centrifugal forces which enhance the wedge action whereby said stresses in the material of the elements increase further. The materials mentioned above, from which the wear-resistant elements are often made, are, however, rather brittle and, therefore, uncapable of sustaining large tensile and/or bending stresses.